400 



Comparative Animal Physiology 



the binocular field is 20-30 degrees horizontally directly ahead, with ex- 

 tremes of 4 to 40 degrees or more. Most birds have binocular fields (Figs. Ill, 

 112), and these are known to vary from 6-10 degrees in parrots to 60-70 de 



Fig. 111. Projections of the four foveas of a hawk through an imaginary surface 



at the distance of momentary accommodation and convergence, forming a "visual 

 trident" of high resolving power. (Courtesy of Dr. Gordon L. Walls.) 



I'M 



Fig. 112. Freezing posture of bittern, showing binocular vision beneath head. Walls."" 



grees in owls. Some birds even have a binocular field of view helow the bill, 

 e.g., a bittern can assume a "freezing" posture with its bill almost vertical and 

 still have a binocular field \'entral to the raised head, i.e., horizontally (Fig. 

 1 12). In simians and in man the binocular field is about 140 degrees, i.e., the 



